I wouldn't put one on any engine I cared about. Others will have contrary opinions. I liked a previous suggestion from X5 in AK, who posted that one option was to remove the filter entirely, and see if you notice any power difference. Any low restriction air filter (whether it is low restriction or not is another subject entirely) is going to give you only a small % of the difference you feel with no filter. So, either put a K&N in or put the original back in. Note that any consequential damage to your engine from running with no filter, or with a filter that performs less than the OEM filter, will cause you to be responsible for engine damage. That includes dusting the engine, or mass airflow problems due to improper oiling of the filter.
I did a quick search on 'K&N filter' and found 81 posts. This was the most recent:
http://www.xoutpost.com/x5-e53-forum/...t=K%26N+filter
You may want to do the same search and see what is there. 
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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